This studyh focuses on thermal comfort and cost effectiveness of stratum ventilation in subtropical Hong Kong Special Adminstation Region (HKSAR). The need for studying thermal comfort with various air distribution strategies becomes a significant issue recently due to climate change, increasing energy prices and the governmental energy efficiency policy. Stratum ventilation, with air supplied at breathing level, can probably provide satisfactory thermal comfort at a relatively elevated indoor temperature in which less energy use is consumed. It seems that only limited studies on the evaluation of neutral temperature, which is a condition of neither slightly warm nor slightly cool, are supported by actual human comfort surveys. Moreover, study on the related thermal comfort and cost effectiveness as other paradigms in comparison with the mixing and displacement air distribution design is rare. New environmental chamber of laboratory-based air-conditioning systems has been developed for investigating the actual benefit of cost effectiveness and balance of thermal comfort satisfaction with the stratum air distribution strategy under subtropical climates. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 7-point questionnaires have been collected from human comfort tests so as to estimate the neutral temperature of stratum ventilation in comparison with mixing and displacement ventilation at pre-set conditions. The neutral temperatures of HKSAR people under the mode of mixing, displacement, stratum, modified-stratum-1, modified-stratum-2, and modified-stratum-3 are found to be 24.6℃, 25.1℃, 25.6℃, 26.0℃, 27.1℃ and 27.3℃ at 10 air change per hour (ACH) respectively, which become 24.8℃, 25.3℃, 26.6℃, 27.4℃, and 27.9℃ at 15 ACH respectively. Life cycle assessment results in 10 service year indicate that 7.73% and 7.32% of cost reduction, and 14.52% and 11.91% of greenhouse gas emission reduction in stratum ventilation by comparing with mixing and displacement ventilation. As a result, stratum ventilation should be the best option on both of cost reduction, and less carbon emission in small-to-medium size air-conditioned space for new building and retrofitting existing works.

Energy use in buildings account for about one third of the total global energy supply and contributes as much as 30% of the anthropogenic greenhouse gas emissions. It is estimated that energy use in buildings will increase to 67% by 2030. The need for better thermal comfort and air quality in indoor environments is the leading cause for high energy use in buildings.  Heating, ventilation and air conditioning systems take up about 50% of the total energy use in buildings which is about 10-20% of the national energy use in most developed countries. The development and adoption of sustainable ventilation systems is a viable solution to mitigate climate change and curtail carbon emissions. The experimental study was conducted in a room resembling a modern office in a laboratory environment. The study involved investigating the ability of the system to provide cooling and heating. Concentration decay tracer gas technique using Sulphur hexafluoride (SF6) gas was used to determine the local air change index and air change efficiency in the room. Low-velocity omni-directional thermistor anemometer type CTA88 were used to measure the air velocity and temperature in the room. Smoke was used to visualise the flow patterns created in the room.  The climate chamber was used to mimic climatic conditions in winter. Fifteen cases were investigated with five air flow rates set points (30, 40, 50, 60 and 70 l/s) at three supply air temperatures, i.e., 17.6 °C, 21.0 °C and 25.3 °C. The results of the local air change index and air change efficiency for the nominal supply temperature of 17.6 °C showed that the system had strong characteristics of a mixing ventilation system. At the supply air temperature of 21.0 °C, the performance of the system deteriorated slightly to below that of a mixing ventilation system and could not satisfactorily provide heating at supply temperature of 25.3 °C. Better performance of the system at all supply air temperature setpoints was observed at lower airflow rates. At all supply air temperature setpoints, relatively higher degree of temperature stratification was observed at lower supply. The draught rate levels decreased with increase in supply air temperature and height. The location of the air inlet terminals in relation to the workstations had significant effect on the performance of the system. The stratum ventilation system did not work efficiently because the air streams were heavily mixed before reaching the occupants.